The Adsorption, Desorption, and Exchange Reactions of Oxygen, Hydrogen, and Water on Platinum Surfaces. I. Oxygen Interaction

Abstract

The kinetics of adsorption and desorption of oxygen on a platinum filament have been studied by temperature-programmed desorption mass spectrometry. Oxygen adsorption becomes significant only after any carbon contamination is removed from the surface. At 300 °K oxygen adsorbs dissociatively into an atomic β-state which contains four overlapping sub-states. The initial sticking probability is 0.16 but this falls off rapidly with increasing coverage. A kinetic analysis for desorption from the β 2 - and β 4 -states gave Edes ≠ (β 2 ) = 39 kcal mol −1 and Edes ≠ (β 4 ) = 54 kcal mol −1 The desorption was first order suggesting a lack of mobility of the adatoms at the desorption temperature (∼700°K). No oxygen atom desorption could be detected in a line-of-sight experiment. Adsorption of a mixture of 32 O 2 and 36 O 2 resulted in complete isotopic mixing on desorption. At 115 °K, the β-state still populates first with the same initial sticking probability indicating adsorption is nonactivated. Moreover, the sticking probability remains at its high initial value for a much more extensive coverage range, suggesting a precursor state to adsorption. After the β 3 - and β 4 -states are fully occupied, further adsorption into the β-state is competitive with occupation of an α-state which desorbs at low temperatures (∼150°K) with first order kinetics and Edes ≠ (α) ∼6 kcal mol −1 No isotope mixing occurred in the α-state which is undoubtedly molecular. Prior population of the β 1 - and β 2 -states at 300 °K reduced the α-state adsorption at 115 °K suggesting that α and β occupy the same sites. Each site can adsorb either an atom or molecule since each molecule added to the β-state excludes two molecules from the α-state.